Photoelectric conversion elements are used in various photosensors, copying machines, solar cells, and the like. These photoelectric conversion elements have adopted various systems to be put into use, such as elements utilizing metals, elements utilizing semiconductors, elements utilizing organic pigments or dyes, or combinations of these elements. Among them, solar cells that make use of non-exhaustive solar energy do not necessitate fuels, and full-fledged practicalization of solar cells as an inexhaustible clean energy is being highly expected. Under such circumstances, research and development of silicon-based solar cells have long been in progress. Many countries also support policy-wise considerations, and thus dissemination of silicon-based solar cells is still in progress. However, silicon is an inorganic material, and has limitations per se in terms of throughput and molecular modification.
As a next-generation technology to solve such problems as described above, research is being vigorously carried out on dye-sensitized solar cells. Particularly, Graetzel et al. at l'Ecole Polytechnique de l'Universite de Lausanne in Switzerland have developed a dye-sensitized solar cell in which a dye formed from a ruthenium complex is fixed at the surface of a porous titanium oxide thin film, and have realized a conversion efficiency that is comparable to that of amorphous silicon. Thus, they instantly attracted the attention of researchers all over the world.
Patent Literatures 1 to 3 describe dye-sensitized photoelectric conversion elements making use of semiconductor fine particles sensitized by a dye, to which the foregoing technology has been applied. These have an advantageous that a device can be produced by relatively simple steps. However, the ruthenium complex dyes used in the sensitized dyes are very expensive. Furthermore, there are concerns about the supply of ruthenium, and it still cannot be said that this technology is satisfactory as a next-generation technology supporting clean energy to cope with the above-described problems in a full-fledged manner. Rather, the research and development intended for practicalization has been just begun to some extent. For such reasons, development of a photoelectric conversion element which is sensitized by an organic material that is inexpensive and is less restricted in resources, and which has sufficient conversion efficiency, is desired. Reports are beginning to emerge on the use of organic dyes as sensitizers of a photoelectric conversion element (see Patent Literature 4).